Polyethylene is one of the most frequently used commercial polymers. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE). LDPE is a versatile polymer which can be used in a variety of applications, such as film, coating, molding, and wire and cable insulation. There is consequently still demand for building new polymerization plants for obtaining LDPE and very often polymerization in tubular reactors is selected as technology for these new plants, especially because it was possible to increase the capacity of such tubular reactor LDPE plants significantly.
A normal set-up for a tubular reactor LDPE plant consists essentially of a set of two compressors, a primary and a high pressure compressor, a tubular polymerization reactor and two separators for separating the monomer-polymer mixture leaving the tubular reactor, wherein in the first separator, the high pressure separator, the ethylene separated from the monomer-polymer mixture is recycled to the ethylene-feed between the primary compressor and the high pressure compressor, and the ethylene separated from the mixture in the second separator, the low pressure separator, is added to the stream of fresh ethylene before it is fed to the primary compressor. Monomer supply to the tubular reactor can either be carried out solely in the beginning of the reactor or only partly in the beginning with the other part fed via one or more side feed entries. Moreover, it is also common to introduce the initiators in multiple places down the tube, thus creating multiple reaction zones. Such a set-up is state of the art and, for example, described for a specific embodiment in WO 2004/108271. It is further common, to use initiator mixtures. WO 2004/078800 describes a method of selecting initiator mixtures with respect to minimum initiator costs, for instance.
The polymerization process in a tubular LDPE reactor is carried out at high pressures which can reach even 350 MPa. Such high pressure requires special technology for the process to be handled in a safe and reliable manner. Moreover, it needs a lot of energy to compress the monomers to the high pressures of the reaction.
The properties and the structure of the obtained LDPE, such as molecular weight, molecular weight distribution and the amount of short- and long-chain branching, depend strongly on the reaction parameters pressure and temperature. That means, control of the reaction conditions is essential. There are of course additional possibilities to influence the nature of the produced LDPE, for example the addition of chain-transfer agents, which reduce the molecular weight, however in general the possibilities to vary the reaction conditions while obtaining a specific target product are quite limited. A further limiting factor for the production of LDPE is heat removal from the reactor, because the heat of polymerization of ethylene is relatively high. That means, for obtaining different grades of LPPE, i.e., polymers which differ in melt flow rate (MFR) and density, it is in general necessary to adjust the operating parameters, which might result in different output rates.
Moreover, if the temperature rises too much in the reactor, there is the risk that the ethylene decomposes to carbon and a mixture of methane and hydrogen, which leads to rapidly increasing pressure in the reactor requiring an emergency shut-down of the plant via a relief valve or a burst disk. However, such situations are highly undesirable.
As long as a polymerization plant is operated in normal mode, the operator will always try to run the plant with full or even higher load, since a major part of the production costs for a polymer are investment costs for the plant and the higher the load of a plant the lower are the proportionate investment costs for the produced polymer. However, there might be exceptional circumstances that require that the output of a polyethylene plant has to be reduced, for example if another facility fails or is shut down. That could be on the side of the feedstock, e.g., an ethylene producing cracker is turned off and it is not possible to compensate, for example, via a pipeline network or from storage tanks; or it could be on the side of the product, e.g., there exists a problem with logistics and the manufactured material cannot be shipped.
It turns out more and more frequently, especially for polymerization plants with capacities of 200 kt/y and more, that flexible ethylene consumption for a limited period of time is needed. There are presently however only limited options. There is of course the possibility to reduce the amount of added initiator. However, if the further reaction parameters are kept constant, the reaction temperature decreases and the properties of the obtained polyethylenes change.
It is further possible to varying the suction conditions of the high pressure compressor, i.e., reducing the ethylene consumption of the compressor. However, that results at the most in a reduction of the output of the plant of about 10%. The temperature of the monomer feed to the tubular reactor can be increased by increasing the pre-heater temperature, which also reduces the output. This measure, however, is practically limited in terms of output reduction to a maximum of about 5%. Furthermore, there is the opportunity to increase the inlet cooling medium temperature to the reactor, which will result in a significantly reduced heat transfer from reactor to reactor cooling media entailing an output reduction of up to 10%. All those measures however imply severe changes of the reactor conditions and need numerous adjustments of to the polymerization conditions to balance those changes and are moreover even combined insufficient. There is of course additionally the possibility to change the produced grades, however that also allows only for a limited reduction of output and it might require production of grades, which are not suited to the market needs at that point of time.
There could be a solution for this problem by constructing new tubular reactor LDPE plants with a set of two high pressure compressors instead of only one. It would then be theoretically possible to run the plant with only one of the two high pressure compressors and consequently cutting the output in half. Some older tubular reactor LDPE plants with relatively low capacity are indeed constructed in such a way. However, the investment costs for a pair of two compressors are much higher than that for only one with the same capacity and moreover it is not even possible to operate a tubular reactor with significantly reduced output is this way. By cutting the monomer intake in half the gas velocity within the tubular reactor is severely reduced, which negatively impacts heat removal and consequently the product properties and furthermore also enhances the risk of spontaneous decomposition of ethylene.